1. Field of the Invention
The present invention relates to an electric vehicle, and more particularly to the technique of controlling running speed of an electric vehicle while maintaining a high degree of safety.
2. Description of the Prior Art
Electric vehicles are attracting attention today as automotive wheelchairs for the aged and those having difficulties of walking. Such a vehicle does not run at high speed as an ordinary automobile does, but runs at a walking speed. With the electric vehicle, therefore, the question of speed control and safety calls for an approach different from that for the ordinary automobile.
In a known technique of controlling running speed of an electric vehicle, an amount of operation of a speed controller such as an accelerator lever is detected with a potentiometer or the like, and the result of detection is used by a control device in determining a power supply to an electric motor.
However, when the vehicle runs on an uphill road, for example, its speed becomes slower, even if the accelerator lever is operated a predetermined amount, than when running on a level ground. When the vehicle runs on a downhill road, the opposite is the case and its speed becomes increased. Thus, there is room for improvement.
Feedback control is well known in the technical field of speed control, in which an actual speed is detected and power supply to the motor is adjusted to cause the speed to follow a target value. This control is suited as an accurate speed control where only minor load variations occur. However, it is not well suited as a speed control where great load variations occur with the motor in running on a sloping road or an unpaved road.